This invention relates to an improved process for making a conductive medium for electrostatic printing and the conductive medium therefrom. More particularly, the invention is directed to an improved aqueous-based process for making a dielectric-coated medium that affords excellent imaging characteristics in electrostatic printing applications.
Electrostatic printing media generally comprise a dielectric layer on an electrically conductive base. The dielectric layer of this combination permits retention of a latent electrostatic image on its surface until the image is developed and fixed by toner. Further, the electrically conductive base is usually constituted to dissipate any stray electrical charges so that the resulting prints will have a non-interfering background. Preferably, a dielectric printing medium would be fabricated by applying the dielectric coating directly to a paper base sheet. However, the electric conductivity of conventional cellulosic basestock varies significantly, low conductivity of basestock producing poor ultimate image reloution and increasing background contamination of the resultant print. Accordingly, it has become conventional practice in the art to pretreat cellulosic base sheets with conductive material before applying the dielectric layer. This pretreatment improves and standardizes the base sheet's electrical conductivity, conductivity being the inverse of resisitivity. Conductive pretreatments may be applied in the form of base sheet impregnations or sheet subcoatings. Conductive materials useful for such pretreatment and their methods of application to the base sheet are well-known in the art. For example, U.S. Pat. No. 3,385,730 discloses an aqueous pretreatment composition of glycerine and ammonium chloride to standardize and enhance base sheet conductivity. Other such compositions and processes are disclosed for example, in U.S. Pat. Nos. 3,075,859; 3,216,853; 3,348,970; 3,493,427; 3,520,771; 3,629,000; 3,639,640 and 3,935,335.
The conductive materials useful in the above-described processes are usually water soluble. Moreover, they retain their water solubility after precoating of the base sheet. Unfortunately, it is difficult to apply water soluble dielectric layers to such water soluble electrically conductive precoats or impregnated sheets without diffusion of some conductive material into the dielectric layer. Yet, conductive contamination of the dielectric layer substantially destroys the dielectric layer's required insulating character and degrades the ultimate printing performance of the medium. Therefore, many prior art dielectric coating processes were carried out in organic solvents to avoid dissolution of the conductive aqueous precoats and the resultant conductive contamination of the dielectric layer. However, organic solvents are more expensive than aqueous ones. They are also more inflammable and hazardous to personnel and the environment. It would therefore be more preferred to use aqueous-based dielectric compositions in any commercial process for making conductive paper. Examples of some available aqueous-based dielectric compositions are disclosed in U.S. Pat. Nos. 3,216,835; 3,348,970; 3,629,000; 3,847,661 and 3,920,880.
Unfortunately, most prior attempts to apply water-based dielectric layers directly on top of aqueous conductive precoats have been unsuccessful in avoiding substantial conductive contamination of the dielectric layer and the other problems related to unfavorable interaction between the two functionally disparate but like soluble layers. E.g., U.S. Pat. No. 3,759,774, column 2, lines 22-37 and U.S. Pat. No. 3,847,661, column 1, lines 23-32.
Several attempts have been made to avoid this debilitating layer interaction through use of a separate intermediate barrier layer to separate the surface of the conductive precoat or impregnation from the dielectric layer. Preferably, the intermediate layer is chosen to be compatible with and receptive to both the conductive layer and the dielectric layer and to provide a good bonding surface between them. One such protective or barrier layer, comprising oxidized starch and calcium carbonate, is disclosed in U.S. Pat. No. 3,759,744. Although this barrier layer substantially prevents conductive contamination of the dielectric layer, its use and process of its application is economically disadvantaged by the additional equipment and material needed to effect intermediate layer formation.
Recognizing this disadvantage in the former separate barrier layer, the coating process of U.S. Pat. No. 3,956,571 relies for its conductive layer-dielectric layer separation on an incidentally formed migration-prevention layer. This incidental layer is formed between the conductive layer and the dielectric layer by an ionic reaction between anionic components of the dielectric composition and cationic components of the conductive precoat. The resultant intermediate layer apparently prevents the migration of the aqueous conductive components into the aqueous dielectric layer.
Also avoiding the disadvantages inherent in separate barrier layers are the processes described in U.S. Pat. Nos. 3,709,728 and 3,672,988. The former is characterized by two distinct applications of a special aqueous dielectric dispersion to the base sheet. Each of the dielectric layers formed in this process becomes water insoluble upon heat aging. More usually, an insolbuilizer, such as a water-soluble melamine formaldehyde resin, is included in the dielectric dispersion to speed the insolubilization and improve the water insolubility of the composite dielectric layer. Subsequent to the water insolubilization of the two dielectric layers, the base sheet is rendered more conductive by impregnation with an aqueous solution of a conductive salt from the side opposite to the dielectric coating. Although this process avoids dielectric layer contamination, second step backside impregnation is not as effective as initial conductive layer precoating or impregnation for producing the desired electrically-conductive substrate preferred for optimum electrostatic printing. In addition, the necessity to use two separate dielectric layers in this process disfavors it economically since the cost of dielectric material is the most substantial factor in the unit cost of the conductive medium.
The latter process is characterized by the use of an aqueous colloidal alumina-resin composition to increase the electrical conductivity of the base sheet. The resin component of this composition dries to form a water-insoluble film that is amenable to subsequent overcoating with conventional aqueous dielectric compositions. Usually, water-soluble polymers or aqueous emulsions of resins capable of forming water-insoluble cured films, such as styrene/butadiene copolymer latexes and butadiene/methylmethacrylate copolymer latexes, are employed as the resin or insolubilizing component in such compositions. On disadvantage of this process is that the alumina-resin layer on the underside of the paper, i.e. that not overcoated with the dielectric layer, displays insufficient hold-out to prevent carrier solvents of conventional developing agents from permeating into the interior of the paper. Such permeation results in inferior electrostatic printing, both in terms of image development and handling characteristics. Another disadvantage is that a resin component in addition to the alumina, the major conductive component of the low-resistance layer, must be employed to attain the required water insolubility of the conductive subcoat.